Normally user equipment (UE) in active mode in a cellular wireless communication system is handed over from one cell to the next as it moves through the network, and data can be transmitted and received without significant interruptions due to these handovers.
The handover (HO) procedure can consist of many steps. In most cellular wireless communication systems the handover is:
1) network controlled, i.e. the UE is commanded by the network when to connect to another cell;
2) prepared, i.e. the target cell (the cell that UE is moving to) is prepared;
3) UE assisted, i.e. the UE provides measurement reports before handover, to the serving cell to assist the decision to do handover preparation of target cell(s), and when to leave the serving cell/connect to the target cell.
In the context of handover, the serving cell before handover is often referred to as the source cell. After successful handover the target cell becomes the new serving cell. In LTE, the handover is a so called “hard handover”, i.e. the UE radio link is switched from one cell (source) to another (target). In universal mobile telecommunications system (UMTS) hard handovers are used exclusively for time division duplexing (TDD) mode and may be used for frequency division duplexing (FDD) mode too.
In the following discussion, see FIG. 1, the focus is on the intra frequency long term evolution (LTE) handover procedure, but the procedures are similar for the LTE inter radio access technology (RAT) and LTE inter frequency handover procedures. The intra evolved universal terrestrial radio access network (E-UTRAN) in RRC_CONNECTED state is UE assisted network controlled handover, with handover preparation signalling in E-UTRAN. The figure below depicts the basic handover scenario where core network nodes (the mobility management entity (MME) and serving gateway (S-GW)) do not change.
The handover is initially triggered by a measurement report sent from the UE to the serving eNB (evolved Node B). The serving eNB configures how the UE shall take measurements (“Measurement Control” step 1 in FIG. 1) and under what conditions a measurement report shall be triggered and sent to the eNB.
To assist mobility control decisions, the UE can measure several different candidate targets cells and report the results to the network. Different networks and network deployments can have different detailed behaviour, but in most networks it is natural to trigger handover when signal reception from a target cell is better than from the source cell (FIG. 2). In the measurement report the UE includes the reason for the trigger (e.g. target cell stronger than serving cell) and measurements of the reference signal strength (RSRP) or quality (RSRQ) of the serving cell and several neighbours (including the target cell). To reduce ping-pong effects where a UE hands over repeatedly between two cells, a handover offset value is often added to the trigger condition: target cell should be better than the serving cell by the handover offset value (offset>0 dB).
When the serving eNB receives a measurement report and if it desires to handover the UE to another cell, it performs a handover preparation to that cell. Handover preparation involves a signalling exchange between one eNB and another eNB. The source cell requests the handover (Handover Request, step 4) and passes over UE context information; the target cell decides if it can admit the UE (Call Admission Control, step 5) and either accepts or rejects the handover. In the acceptance message (Handover Request Ack., Step 6) the target cell includes parameters required by the UE to allow it to communicate to the target cell these parameters are grouped into a transparent container.
Following a successful preparation, the handover execution takes place. The source cell issues the HO Command to the UE—this is the RRCConnectionReconfiguration message and carries the transparent container. If and when the UE receives this correctly it synchronises to the new target cell and sends a synchronisation message on the random access channel (RACH, step 9). The target cell then issues an allocation to the UE (step 10) so that it can send a HO Confirmation message to the target cell (the RRCConnectionReconfigurationComplete message, step 11).
The final steps, the Handover Completion, do not involve the UE. The source eNB is able to forward data (unacknowledged downlink packets) to the target eNB, and the S1-U interface from the S-GW must be switched from the source to the target (“path switch”). Finally, if the handover is successful the target eNB issues a UE Context Release message to the source eNB.
A successful handover requires (see FIG. 1):                Measurement report delivery from the UE to the serving cell, followed by handover decision at the serving cell;        Communication over the X2 between the serving and target eNB (HO preparation);        Delivery of the HO Command RRC message from the serving cell to the UE;        Successful random access and delivery of a HO Confirm RRC message to the target cell.        
Failure of a handover can occur at any of these stages. The transmission of the RRC signalling to/from the UE is managed by the RLC AM protocol and this judges when failure has occurred (and attempts to transmit the message should be ceased). Additionally, the UE performs measurements of the quality of the downlink of the serving cell and can determine a radio link failure at the physical layer level when the quality is judged to be poor for a duration equal to a timer value, “T310”. Since the “offset” is typically assumed to be greater than zero (otherwise ping-pong handovers between source and target are very likely), the majority of failures are expected to occur in the source cell before the HO Command has been delivered—the HO begins when the UE is already closer (in radio terms) to the target cell than to the source, and the UE moves further from the source as the handover proceeds. This generalization may be broken by randomness in the UE motion and by fragmented coverage caused by shadowing.
The UMTS hard handover is very similar in many respects—it is also UE assisted but in this case network controlled (the UE is configured to send triggered measurement reports but the network decides when to execute a handover), exploits preparation (using RL Setup procedure), is a “backward” handover (the source cell sends the HO command to the UE and the UE replies to the target cell) and is completed by inter-node signalling.
Handover algorithms are concerned with two performance aspects above all, namely:                Failures during the handover, or before the handover has been triggered; and        Number of handover events, however less important than the case above.        